The present invention is generally directed to a system for processing acoustic signals generated and received during the course of ultrasonic imaging. More particularly, the present invention is directed to a digital phase correction circuit which employs an architecture which is particularly suitable for use in ultrasonic imaging machines, particularly those employed in medical applications.
In the formation of images generated by ultrasound systems, it is necessary to be able to selectively view individual points within a body being imaged. Each such "point" actually comprises a small volume element whose dimensions depend upon the resolution of the overall system. However, various internal bodily structures exhibit collections of such points with different sonic absorption and reflection capabilities. It is these differences which are employed to generate the desired image. Nonetheless, it is necessary to be able to collect information about the sonic properties of these volume or pixel elements.
In a conventional ultrasonic imaging system, a linear array of transducers is provided. This array is typically fabricated in the form of a wand which is moved over the body of the patient by an ultrasonic imaging specialist. The transducers typically serve a dual function. Not only do they produce the ultrasonic wave fronts within the body, but they also operate to receive ultrasonic echo signals which emanate from various internal bodily structures which have been ultrasonically excited.
In a typical ultrasonic imaging device, an ultrasonic wave front is generated within the body being investigated. Subsequent to the excitation, a linear array of transducers receives echo signals from all of the points that have been excited. It is the object of the present invention to be able to electrically "orient the receiver" so as to be able to produce a signal which is proportional to the amplitude of the echo signal from a single reflection point (voxel or pixel) within the body. In this regard, it must be borne in mind that the signal from such a point travels different distances to the different transducers in the array. In so doing, this signal (from a single reflection point) is delayed by a time proportional to the distance between the point and a given transducer in the array. Naturally, this delay time is a function of the position of the transducer in the array and also a function of the position of the point. Of particular importance is the fact that the signals from a single reflection point arrive at the transducer array in such a manner as to exhibit phase differences arising from distance differences. These distance differences also produce time delay differences. Furthermore, because the ultrasonic echo signals must traverse different length paths and because there is a certain degree of attenuation which is generally proportional to the path length, the echo signals arriving at the various transducers also exhibit amplitude differences as a result of distance differences between the reflection point and the various transducers in the array. It is noted though that relative timing control as between various transducers in the array may be employed to gate all signals from a single reflection point through the transducer array at a single time. This effectively permits the array to "look at" a single observation point. Additional timing control permits the transducer array to be "focused" at an observation point which is a certain distance from the array, as determined by additional timing control. Naturally, the transducer array receives signals from other excited points at the same time. Nonetheless, it is possible to form the summation of the signals from a transducer array steered to and focused upon a single point so that, on the average, the signal from that point dominates signals from all other points in the view plane. Ultrasonic imaging systems typically provide beam steering and focusing by generating phasing, time slipping and time delay corrections for the received signals. Phasing is generally generated by providing approximately 128 differently phased clock signals to 64 receivers. Time delay is provided by use of variable length delay lines. Some systems also provide time slipping by digitizing the signal at a frequency of 20 megahertz and using the sample closest to the desired time. In this fashion, an ultrasonic beam is steered and focused. Nonetheless, the problem of phase differences still exist.
It should be noted that digital phase correction becomes more and more necessary at higher operating frequencies for ultrasonic transducers. For example, suppose that a phase quantization of one part in thirty-two at an operating frequency of ten megahertz is required. An analog phase slip in the mixers would require a 3 nanosecond time resolution. This is difficult, if not impossible, and even if possible, would be so only at great expense which would probably be prohibitively high. With the digital phase correction of the present invention, however, this is not a problem and the desired 1/32 phase correction is readily achieved.
While current ultrasonic imaging systems are easy to use and have been found to be extremely beneficial in medical diagnostic applications, it is still nonetheless desirable to be able to reduce the cost and size of such systems so as to make them even more available and more effective as diagnostic tools. In particular, it is desirable to be able to employ very large scale integrated circuit methods and architectures in the structuring of ultrasonic imaging systems. Such systems would alleviate the need for employing a large number of discrete electronic components disposed on a relatively large circuit board and requiring interconnection of the discrete electronic components.